1. Field of the Invention
The present invention is directed to an epoxy resin encapsulating composition with enhanced moisture resistance for semiconductor packages and the method for producing the same. More particularly, the present invention is directed to such an epoxy resin encapsulating composition which includes an inorganic ion-exchange solid material for trapping particularly the chlorine ions originating from an epichlorohydrin forming the epoxy resin and inherently present in the epoxy resin.
2. Description of the Prior Art
In recent years, there has been a growing demand for miniaturizing semiconductor packages in the form of, for example, a small outline package (SOP) and a flat package (FP) which are capable of being directly mounted on a circuit board with external solder dip leads thereof being soldered to the circuit board. In this direct mounting process, it is a standard industry practice to dip entire package in a soldering bath and then place it on the circuit board for simultaneous soldering of the external leads to the corresponding lands on the circuit board. Due to this recent industry demand of dipping the entire package in the soldering bath prior to the mounting thereof, increasing emphasis is being placed on adding enhanced moisture resistance to an epoxy resin encapsulating composition for the semiconductor packages, or on utilizing the epoxy resin encapsulating composition which has an enhanced moisture resistance which remains intact even when it is dipped in the soldering bath to be subjected to intense heat and vapor of the solder. Such enhanced moisture resistance is also required for the packages which are exposed in the actual operating condition to an elevated temperature environment.
In the meanwhile, it is known that epoxy resins made from epichlorohydrin and an aromatic hydroxyl-containing compound inevitably contain a certain amount of undesirable by-products in which the chlorines of the epichlorohydrin are coupled with certain epoxy groups of the resulting epoxy resin. This undesirable by-product is very likely to be hydrolyzed when exposed to heat in the presence of moisture into the chlorine ions which show strong corrosive action as reacted with the moisture, eventually corrode the metallic conductor of the semiconductor such as aluminum conductor so as to damage the semiconductors or reduce the moisture resistance of the package thereof. In order to avoid such unacceptable reduction in the moisture resistance, there have been attempted to reduce the amount of the by-product contained in the epoxy resin adduct. However, this approach involves complicated and difficult treatments which are not practically acceptable and do not assure complete removal of the undesired by-product. In this respect, the remaining by-product will act to lower the moisture resistance as it is hydrolyzed into the corrosion inducing ions when the package or epoxy resin encapsulating composition is subjected to heat and moisture at the soldering process or in a high temperature operating environment. Further, there is another problem that the package when subjected to soldering process will suffers from the permeation of the chlorine ions Cl.sup.- possibly contained in a flux employed in the soldering through a minute gap developed between the external leads and the molded composition due to the heat stress applied thereto at the time of the soldering process. To this end, it is desirable to utilize a suitable ion exchanger which is capable of trapping the corrosion-inducing ions occurring in the packages after the encapsulation from the standpoint of successfully preventing the lowering of the moisture resistance due to such undesirable ions.